Communication networks enable data to be transmitted between a transmitter and a receiver by utilizing a communication resource. In the case of a wireless communication network, the communication resource is a radio frequency that is manipulated using certain modulation and coding schemes to convey the data. As the utilization of the radio spectrum increases, such communication resources become increasingly congested and scarce.
A “white space” spectrum environment enables wireless communication to be performed using unused parts of the radio spectrum. For example, a primary user can have licensed access to a portion of the radio spectrum, but secondary, unlicensed, users may be able to opportunistically use this portion of spectrum at times when the primary user is not utilizing it. Therefore, a white space environment can ease communication resource scarcity. For example, the FCC has allowed parts of the UHF spectrum below 700 MHz to be used for this purpose.
However, the use of white space environments bring their own challenges. Firstly, it can be difficult to detect and avoid channels occupied by primary users, with whom interference is to be avoided. Secondly, white spaces provide a potentially large pool of available frequency channels (including frequency fragments available between primary users). Due to the wide frequency range (for example up to 200 MHz), the qualities of the available channels can vary substantially.
As a result, choosing a communication resource from the white space (in terms of a frequency channel and transmission rate) is difficult. Neither the received signal strength indicator (RSSI) nor signal to noise ratio (SNR) is a good predictor of channel quality. As a result, probing is used to learn the quality of each channel available at each transmission rate. In other words, several packets are transmitted at each channel and at each rate, to construct a reliable estimate of the channel quality.
However, because of the large pool of available frequency channels (the state space) there are a very large number of channel and rate combinations available for use. Therefore, probing each of these combinations is inefficient. For example, if an optimum channel and rate is to be selected by probing all combinations, then by the time they are all probed and the optimum selected, the conditions may have changed and the selected combination may no longer be optimum. A similar situation can occur in the case of a smaller number of combinations, but more rapidly changing channel conditions. This is an exploration (probing) versus exploitation (utilization) trade-off as a result of the large state space. The transmitter aims to exploit the optimum channel and rate to send data, whilst constantly exploring whether the optimum channel and rate has changed. Exploration involves a cost in that bad or suboptimal channels and rates may be explored which wastes time and communication resources.
The embodiments described below are not limited to implementations which solve any or all of the disadvantages of known channel and rate selection techniques.